1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
As an electrophotography image forming apparatus, a tandem type image forming apparatus having a plurality of photosensitive elements for forming images with different colors from each other has become popular. In this tandem type image forming apparatus, an electrostatic latent image is formed by optical writing on a photosensitive element and a toner image formed by developing this electrostatic latent image is transferred to a printing medium or an intermediate transfer body. This transfer operation is performed for each color, and the toner images of the colors are overlapped on the printing medium or the intermediate transfer body, thereby providing a full-color toner image. In the case of having transferred the toner image to the intermediate transfer body, the toner image is further transferred from the intermediate transfer body to the printing medium. The toner image transferred to the printing medium is fixed on the printing medium, thereby providing the color image.
The positional deviation among the color transfer positions in such an image forming apparatus leads to the deterioration in image quality of the printing image because the color toner images are not overlapped with each other correctly. Therefore, it is necessary to correct the positional deviation among the color transfer positions.
As a method of correcting the positional deviation among the color transfer positions (hereinafter referred to as the positional deviation for short) in the electrophotography image forming apparatus, a method is known in which a positional deviation correction pattern is formed on an intermediate transfer body, a carriage belt for conveying a printing medium, or the like and the positional deviation correction is performed based on the positional information obtained by detecting the position of the positional deviation correction pattern with a sensor.
Japanese Laid-open Patent Publication No. 2011-112704 discloses an image forming apparatus including toner of two special colors in addition to general toner of four colors. In this image forming apparatus, the positional deviation correction pattern to be formed varies depending on the number of colors.
If a pattern image is formed based on one color mode out of mode performing color match control with only four colors and mode performing color match control with five or more colors, a pattern image is formed in the other color mode based on the formed pattern image. As a result, since the number of toner is different, the total pattern length varies. In that case, even though the positional deviation correction is performed based on the patter image formed according to one color mode, the total pattern length is different in the other color mode, in which case the positional deviation correction cannot be performed as appropriate.
Therefore, it is desirable to provide an image forming apparatus capable of performing positional deviation correction as appropriate on any of a plurality of color modes.